An instant messaging service enables users who are online to instantly transmit and receive a message each other. The instant messaging service is provided based on e-mail accounts of the users on a wired Internet. The instant messaging service provides a one-to-one chatting feature, a file transmitting and receiving feature and other features.
With development of a mobile communication technique, recently a mobile instant messaging service is provided based on a mobile communication network. The mobile instant messaging service is derived from application of the wired Internet-based instant messaging service to a mobile communication system. A mobile communication subscriber installs an instant messaging client intended for a mobile terminal in his/her mobile terminal and uses an instant messaging service used in a wired Internet as it is.
A wire/wireless instant messaging service is not monopolized by a single service provider, but competitively provided by a plurality of service providers. For example, the wire/wireless instant messaging service includes MSN messenger by Microsoft, Daum messenger by Daum communications or NateOn messenger by SK communications.
The wire/wireless instant messaging services of the service providers are currently incompatible with each other, and attempts at creating a unified standardization have failed. As a result, in the case that an instant messaging service user uses a different messenger program from a contact with who he/she wants to chat, the instant messaging service user should download a messenger program of the contact that is provided by a different service provider from that of his/her existing messenger program.
In particular, a mobile communication terminal has lower data processing speed and memory capacity than a wired terminal, and thus an instant messaging service using the mobile communication terminal has difficulty in installing all of different instant messenger programs in the mobile communication terminal due to a limited performance of the mobile communication terminal.
Meanwhile, 3GPP (third generation partnership project) adopted a system structure, in which an IP multimedia (IMS: IP Multimedia Subsystem) domain for complexly providing multimedia including voice, audio, video or data based on IP protocol is introduced to a mobile communication backbone network. With introduction of a WCDMA (Wideband Code Division Multiple Access) system, the communication service providers have recognized the IMS domain as a core technology leading a market, and attempted to provide the mobile instant messaging service in the IMS domain.